It is a known procedure to aftertreat metal articles and metal surfaces with chromate in order to obtain a surface coating having corrosion-protecting and/or decorative properties. The treatment is called a chromate treatment and is known for instance in connection with zinc-coated, cadmium-coated or silver-coated copper or iron, including steel. Conventionally, aluminium and aluminium alloys are also treated by a chromate treatment.
The protective effect of a chromate treatment is due to a chemical conversion of a thin metallic surface layer of zinc, cadmium, silver or aluminium by reaction with chromic acid or chromates to form chromium hydroxide/chromate, The resulting layers have also been found useful in treating metal surfaces which are corrosion-resistant per se as such layers are distinguished by being very thin and can be used for achieving a particularly decorative effect.
Although the chromate treatment has the advantages of excellent anticorrosive and decorative properties, and although the method is simple and inexpensive, the use of chromate is restricted by the regulations applying to environmental pollution, and chromate causes problems, such as toxicity to the workers exposed to chromate during the treating process, and difficult disposal of the chromate sludge after the precipitation from the spent solution. In addition, a possibility exists of chromate being released from the chromate-treated products.
Buttner, Jostan and Ostwald, Galvanotechnik 80 (1989) No. 5, pages 1589-1596, have tested various methods for their applicability as replacements of the chromate treatment. Among these possible methods, Buttner et al mention formation of layers containing molybdenum and tungsten by treatment with molybdenum and tungsteniso- or heteropolyacids or salts thereof in connection with zinc-coated surfaces. The isopolyacids form polymeric anions with the same metal atom, such as HW.sub.6 O.sub.21.sup.5-. The heteropolyacids are formed from the isopolyacids with mineral acids and provide mixed anions, such as P(W.sub.12 O.sub.40).sup.3-. Molybdic acid H.sub.2 MoO.sub.4, ammonium heptamolybdate (NH.sub.4).sub.7 Mo.sub.6 O.sub.24.4H.sub.2 O, molybdatophosphoric acid H.sub.3 [P(Mo.sub.3 O.sub.10).sub.4 ].XH.sub.2 O, ammonium paratungstate (NH.sub.4).sub.10 H.sub.2 W.sub.12 O.sub.42.XH.sub.2 O, phosphotungstic acid H.sub.3 [P(W.sub.3 O.sub.10).sub.4 ].XH.sub.2 O, and silicotungstic acid H.sub.4 [Si(W.sub.3 O.sub.10).sub.4 ].XH.sub.2 O are examples of such compounds which according to Buttner et al. can form molybdenum-containing or tungsten-containing layers on zinc coatings. The layers are precipitated from 2% solutions at room temperature and in some cases with addition of small amounts of acid, base or oxidation agents, such as hydrogen peroxide or sodium perborate. In connection with phosphomolybdic acid, a 2% solution contains approximately 12 g/l of molybdenum and approximately 0.3 g/l of phorphorus corresponding to a molar ratio Mo/P of 12.9. The tests performed by Buttner et al. demonstrate that the materials in question provide a certain passivating effect, but the protecting effect thereof cannot compete with the effect of conventional chromate treatment.
GB-PS No. 1,041,347 discloses a process and a treatment solution for corrosion-protection of metal surfaces, where for instance Example 2 of This publication describes the treatment of steel or zinc- or cadmium-coated steel. This Example uses a first treatment solution containing 0.5 to 2.5% by weight of anionic polymer in form of polyvinyl toluene sulfonic acid of a molar weight of for instance 400,000, 0.1 to 0.5% by weight of zinc carbonate, 0.1 to 0.5% by weight of ammonium molybdate corresponding to from 0.49 to 2.44 g/l Mo, 0 to 0.2% by weight of phytic acid, and 0to 0.5% by weight of orthophosphoric acid, and with a pH-value within the range 5.0 to 6.8, and where the temperature of the treatment solution is maintained at about 125.degree. F. which corresponds to 51.degree. C. A typical treatment solution contains 0.25% by weight of ammonium molybdate and 0.2% by weight of orthophosphoric acid, which corresponds to a molar ratio Mo/P of 2.58. However, the known treatment solution is not used alone as the metal surface is to be treated subsequently with a second treatment solution containing an organic cationic substance reacting with the artionic polymer. Accordingly, the treating process is rather complicated.
GB Patent Application No. 2,070,073 (Kobe Steel Ltd.) discloses an anticorrosive treatment for preventing white rust on galvanised steel, where a solution is applied onto the surface of the galvanised steel. This solution contains molybdic acid or a molybdate in a concentration of 10 to 200 g/l calculated as molybdenum and is adjusted to a pH-value of between 1 and 6 by the addition of an organic or inorganic acid, preferably phosphoric acid.
By proceeding on the basis of the disclosure of the above GB Patent Application No. 2,070,073, it is, however, not possible to obtain a completely satisfactory protection against white rust. It appears thus from the following comparison Examples B and C, that this method results in a substantially poorer protection against corrosion than the protection obtained by the conventional chromate treatment.
EP-A-004501 (Nippon Kinzoka Co. Ltd.) discloses an anticorrosive treatment of stainless steel sheets having a Bright Annealing film or a passive film; the sheet is dipped in a solution containing 0. 1 to 70% by weight of phosphoric acid, 0.1 to 10.0% by weight of a molybdate or chromate or a mixture thereof and 0.1 to 0.2% of magnesium oxide, sodium silicate or mixture thereof; and cathodic treatment is carried out under the conditions of 1 to 600 As/dm.sup.2 integrated electric current density and 0.degree. to 90.degree. C. temperature.
The ranges for the amounts of phosphoric acid and molybdate are rather broad, but only exemplified with 9.37% by weight and 1.34% by weight, respectively in Example 3 giving a molar ratio Mo/P of 0.068 and 10% by weight and 0.5% by weight, respectively, in Example 8 giving a molar ratio Mo/P being even lower.
It is clearly stated and confirmed by experiment that a film (BA or passive) must be formed before the treatment, otherwise there may be no improvement in corrosion resistance (p, 4, lines 11 to 29).
It turned out surprisingly, that it is possible to obtain a corrosion-protecting and decorative coating, which can compete with the .coating obtained by the conventional chromate treatment by means of a treatment solution containing a molybdenum compound (a) and a compound (b) capable of forming a heteropolymolybdate together with molybdate, where the molybdenum concentration and the ratio between the compounds (a) and (b) differs from those suggested by Buttner et al. and from the GB Patent Application No, 2,070,073 and EP-A-O 045 017.